Technical Abstract:
Lignin remains one of the most significant barriers to the efficient utilization of cellulosic substrates, either for pulping or for biofuels production. Now that monomer substitution in the lignification process is now well authenticated in various transgenic plants, it is opportune to begin explorations into designing lignins to improve the ease with which they can be removed from the cell wall. Here we reveal the logic behind our approach to utilize coniferyl and sinapyl ferulate (CA/SA-FA) as lignin “monomers”. These conjugates, ferulate analogs of the monolignol p-coumarates that are incorporated naturally into grass lignins, result in ester linkages connecting polymer moieties; unlike p-coumarates, ferulates incorporate integrally into the polymer by the combinatorial array of radical coupling reactions that typify lignification. The resulting “zips” in the polymer are then readily cleaved by alkaline or acidolytic processes. Incorporation of 25% CA-FA into lignin in suspension-cultured corn cell walls allows delignification at just 100 °C to the same level as requires 160 °C on normally-lignified material; at 65% CA-FA (a probably unrealistic level) just 30 °C is required. Engineering plants to incorporate such monomer conjugates therefore has the potential to vastly reduce the energy demands of processing. With the gene required to biosynthesize the conjugate almost in hand (?), the next step is to attempt the plant engineering.